Salvage of #93649 (@TurgutKural). _apply_pending_fleet_restart_catchup took two
booleans (respect_no_gateway_restart + no_gateway_restart) that were only ever
true together; one `defer` keyword says the same thing. The 13 tests are cut to
the two invariants (pulled path skips restart + verify and keeps the marker;
already-current path defers the catch-up). The user guide gains a section on
running `hermes update` from inside the gateway.
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